1. Field of the Invention
The present invention relates to an optical gate device, a manufacturing method for the device, and a system including the device.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional optical gate device. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is switched out from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having two (both) end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 μm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional optical gate device. The NOLM includes a first optical coupler-including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
The MZI type optical gate is excellent in size reduction and integration, but its manufacturing technique has not yet been established.
The optical gate having an SOA as the nonlinear optical medium has a problem that amplified spontaneous emission (ASE) noise added by the SOA has an adverse effect on fundamental characteristics including a signal-to-noise ratio (SNR).
On the other hand, the NOLM requires a long fiber to obtain a required nonlinear optical effect. Accordingly, there arises a signal rate limit due to chromatic dispersion, and it is difficult to cope with polarization dependence of an input optical signal and polarization fluctuations in the loop optical path.